GB2235577A

GB2235577A - Piezoelectric assembly

Info

Publication number: GB2235577A
Application number: GB9012454A
Authority: GB
Inventors: Wolfgang Geiss; Konrad Voigt; Gunter Helke; Eike Wehrsdorfer
Original assignee: HERMSDORF KERAMIK VEB
Current assignee: HERMSDORF KERAMIK VEB
Priority date: 1989-08-28
Filing date: 1990-06-05
Publication date: 1991-03-06
Anticipated expiration: 2010-06-05
Also published as: NL9001736A; GB9012454D0; DE4014526A1; DD287357B5; GB2235577B

Abstract

A process for producing a stack of laminar piezoceramic elements, wherein the piezoceramic elements have a surface roughness of Rm < 5 mu m. and Ra 0.15 to 0.5 mu m., electrode layers, formed from a relatively hard conducting material, are deposited on a face of each element, each layer extending to an edge of the element, the elements are formed into a stack by repeating a process of applying an interrupted layer of adhesive, having a viscosity of 100-500 mPa.S, to a face of a first element, pressing a second element onto the adhesive coated face of the first element, with an electrode layer sandwiched between said elements, and holding the elements pressed together under a pressure of 0.5-1 mPa until the adhesive hardens and, thereafter, forming electrical contacts connected to the electrode layers on a side of the stack.

Description

A PROCESS FOR PRODUCING AN ASSEMBLY OF LAMINAR PIEZOCERAMIC ELEMENTS AND

AN ASSEMBLY PRODUCED THEREBY

DESCRIPTION

The present invention relates to a process for producing an assembly of laminar piezoceramic elements and an assembly produced thereby.

In particular, the invention concerns a process for producing a stack of laminar piezoceramic elements in which the individual elements are laminated together with interleaving layers of additional binding agents, such as adhesives, under the action of pressure. Such assemblies are used in control elements for causing the mechanical displacement of components in precision apparatus, such as optical and electron microscopes, precision valves and integrated circuit manufacturing equipment, in response to voltages applied thereto.

In DE 3 218 576 there is disclosed a method of manufacturing such an assembly from piezoceramic discs. Control elements which include such assemblies of piezoceramic elements are also disclosed in U.S. 4 488 080. In the process disclosed in DE 3 218 576, individual discs are provided on their flat faces with contacting layers of conducting material (electrodes) by sputtering, evaporative or burning processes (providing either thick or thin layer electrodes). The electrodes are connected up so that a potential difference may be applied across each and all of the discs in the stack. The individual discs-are joined together with an adhesive, and metal intermediate layers, which contact the electrodes, are embedded in the adhesive to provide outwardly extending conductors to facilitate electrical connections to the electrodes.

(DE 3 218 576). The stack of discs is assembled under mechanical pressure, which may be maintained in the finished control element.

It is known to extend the electrodes out to the radial peripheries of the discs, in order to allow J - 3 their connection without the need for any intermediate metal layers.

Most heretofore known processes and arrangements for manufacturing piezoceramic assemblies, including those known from DE 3 218 576, suffer at least some of the following disadvantages:

1.

is Disadvantages resulting from the use of metal intermediate layers: the large non-active mass of the metal layers impairs (reduction) the stiffness and the resonance frequency (reduction in the maximum regulating speed) of the assembly in a direction orthogonal to the ceramic elements; increased assembly costs; prevention of transverse contraction of the discs; 2. Disadvantages resulting from the use of thick layer contacts (electrodes): the relatively large non-active mass of these layers gives rise to similar disadvantages to those caused by the use of metal intermediate layers, especially in the case of very thin discs I- (d:50.2 mm.); the low hardness of the thick layer (which is formed from Ag/Pd) also leads to reduced stiffness orthogonal to the ceramic elements; prevention of transverse contraction of the discs.

3. Disadvantages resulting from the use of adhesive layers:

the high temperature related expansion coefficient of the adhesive prevents consistent expansion or contraction of an assembly and renders it less useful in longitudinal measurement techniques; a reduction in stiffness is also noted.

01 4. Disadvantages resulting from the use of thin layer contacts:

- in theory the use of thin layer electrodes should give rise to an extremely small non-active mass and a reduction in the hinderence to transverse contraction of the elements.

1 1 However, the surface roughness of the ceramic elements is reproduced in the surface of the thin layer electrodes, thus requiring the insertion of metal or adhesive layers between the elements, in order to prevent damage to the elements and electrodes resulting from contact between raised surface features on adjacent elements.

It is an object of the present invention to provide a convenient process for the production of an assembly of piezoceramic elements, which does not suffer from all of the aforementioned disadvantages.

It is a further object of the invention to provide a process which provides an assembly of ceramic elements, the elastic behaviour of which (stiffness, E modulus) closely matchep the ideal. as represented by a monolith of ceramic material. In the production of the assembly Or stack, the individual elements are connected with one another via previously formed thin layer electrodes and additional binding agents, especially adhesives, under the action of pressure.

L According to the invention, piezoceramic elements with a surface roughness of R m <5 pm.

(maximum difference between surface peaks and troughs) and Ra = 0.15 to 0. 5 pm. (average of the distance between surface peaks and troughs) are used. Electrode layers of a relatively hard material, such as Cu/Ni, are deposited on both sides of each element by a known sputtering process, each electrode layer extends up to an edge of the element on which it is formed. Subsequently, an interrupted layer or grid of adhesive with a viscosity in the range of 100 to 500 mPa.s is aplied to an electrode carrying face of a first element and, immediately thereafter, a second element is pressed onto the adhesive coated surface of the first element, under a pressure in the range of 0.5 to 1 HPa, which is maintained until the adhesive has hardened. This process is repeated until a plurality of elements are so assembled. After the last element has been stuck onto the stack, the edges of the electrodes and elements are at least partially coated with electrode material, by means of a sputtering process, to provide contact surfaces.

IL According to the invention. because the adhesive is applied in an interrupted layer or grid, an unbroken film of adhesive does not form between adjacent elements in a completed stack. The spaces rsulting from the surface roughness of the elements alone are filled with adhesive. An "intimate" element/element (ceramic/ceramic) contact is thereby achieved and the negative properties of an adhesive intermediate layer avoided. Pressure needs to be applied while the adhesive hardens, not only to assist the action of the adhesive but also to ensure that there is intimate contact between the ceramic element surfaces. The pressure may be maintained after completion of the assembly.

A particular embodiment of the present invention will now be described by way of example only.

This embodiment relates to the production of a microtranslator using piezoceramic discs formed from a PZT ceramic.

The discs have the following roughness characteristics:

Maximum difference 3.7 pm.

average difference Ra = 0.41 pm.

ip The discs have a diameter of 10 mm. and a thickness of 0.5 mm.

on each side of each disc an electrode consisting substantially of CuNi and having a thickness of 0.8 pm, is formed by sputtering. Each electrode extends up to a portion of the edge of the disc upon which it is formed, in order to facilitate the formation of contacts on the final assembly of discs.

A grid-like layer of a suitable adhesive, e.g. an epoxide resin of cyanoacrylate, is applied to one side of a first disc. The adhesive used has a viscosity of 400 mPa.s. Immediately thereafter, the next disc is pressed onto the first under a pressure of 0.5 mPa, exactly covering the adhesive coated surface of the first disc. This pressure is maintained until the adhesive has hardened.

1 - 9 After a stack of piezoceramic discs has been formed by repeating the aforementioned process steps, the portions of the electrodes adjacent to the disc edges are strengthened to a thickness of about 1.8 lim by sputtering on further electrode material. The strengthened edges provide contact surfaces for electrical connections electrodes.

is A translator produced according to the aforementioned method and in accordance with the invention compares favourably with a similar translator produced by previously known methods.

invention conventional device applied voltage achieved longitudinal change diameter of the discs stiffness of the stack (in axial direction) 1000 v 12 pm. 10 mm.

1000 v gm. 10 mm.

N111m 70 N1Pn.

- 10 The stack or assembly according to the invention (without an armature) is almost as stiff as a similar size monolith of the ceramic material forming the discs and is much stiffer than such a conventional stack:- - monolith material 290 N/gm.

stack according to the invention 200 - 290 N/9m.

stack with thick layer electrodes and metal intermediate layers 100 N/gm.

U

Claims

- 11 CLAIMS

1. A process for producing an assembly of laminar piezoceramic elements in which the individual elements are laminated together in a stack, wherein the piezoceramic elements have a surface roughness of % < 5gm. and Ra -0.15 to O.Sgm., electrode layers, formed from a relatively hard conducting material, are deposited on a face of each element. each layer extending to an edge of the element, the elements are formed into a stack by repeating a process of applying an interrupted layer of adhesive, having a viscos ity of 100-500 mPa.S, to a face of a first element, pressing a second element onto the adhesive coated face of the first element, with an electrode layer sandwiched between said elements, and holding the elements pressed together under a pressure of 0.5-1 mPa until the adhesive hardens and, thereafter. forming electrical contacts connected to the electrode layers where they extend to the edges of the elements,, on a side of the stack.

- 12 2. A process as claimed in claim 1, wherein electrode layers are formed on both faces of each element.

3. A process as claimed in either of claims 1 or 2, wherein the adhesive is applied to each element the form of a grid.

4. A process as claimed in any of claim 1-3.

wherein the electrode layers are formed from a Cu/Ni alloy.

A process as claimed in any of claims 1-4, wherein the electrode layers and electrical contacts is are formed by a sputtering process.

6. A process as claimed in any of claims 1-5, wherein the electrode layers are formed with a thickness of 0.8gm.

7. An assembly of laminar piezoceramic elements laminated together in a stack, wherein the piezoceramic elements have a surface roughness of Rm < 5gm and Ra = 0.15-0.5gm, an electrode layer z 1 1 A 11-1i of a relatively hard material is formed on at least one face of each element,, extending up to an edge thereofi, the elements are held together in intimate contact by an adhesive located only in spaces between adjacent elements which result from their surface roughness and. electrical contacts are formed on a side of the stack in contact with the electrode layers to allow electrical connections to be made to the layers.

8. An assembly as claimed in claim 7, wherein electrode layers are formed on both faces of each element.

9. An assembly as claimed in either of claims 7 or 8, wherein the electrode layers are formed from a Cu/Ni alloy.

10. An assembly as claimed in any of claims 7-9, wherein the electrode layers are 0.8gm thick.

11. An assembly of laminar piezoceramic elements whenever produced by a process as claimed in any one of claims 1-6 and 12.

if - 14 12. A process for producing an assembly of laminar piezoceramic elements substantially as herein described.

13. An assembly of laminar piezoceramic elements substantially as herein described.

14. A microtranslator comprising an assembly of laminar piezoceramic elements as claimed in any of claims 7-11.

Published 1991 at The Patent Office. State House. 66/71 HfghHolborn. London WCIR41?. Further copies rnay be obtained from Sales Branch. Unit 6. Nine Mile Point. Cwmfelinfach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques ltd. St Mary Cray. Kent